A technique in which a transistor is formed using a semiconductor layer formed over a substrate having an insulating surface is known. For example, a technique in which a transistor is formed over a glass substrate using a thin film containing a silicon-based semiconductor material and applied to a liquid crystal display device and the like is known.
A transistor used for a liquid crystal display device is mainly formed using a semiconductor material such as amorphous silicon or polycrystalline silicon. Although a transistor formed using amorphous silicon has low field-effect mobility, it can be formed over a larger glass substrate. Meanwhile, a transistor formed using polycrystalline silicon has high field-effect mobility; however, it needs a crystallization process such as laser annealing and it is not always suitable for a larger glass substrate.
An oxide semiconductor has attracted attention as another material which can be formed over a substrate having an insulating surface and used for forming a transistor. As a material of the oxide semiconductor, zinc oxide or a material containing zinc oxide as a component is known. Thin film transistors formed using an amorphous oxide (an oxide semiconductor) with an electron carrier concentration of lower than 1018/cm3 have been disclosed (see Patent Documents 1 to 3).